CNC Machining Providers
CNC machining provides the dimensional accuracy and surface finish that as-built AM parts cannot achieve alone. Critical interfaces, sealing surfaces, and threaded features are typically CNC-machined after printing. Multi-axis CNC enables complex finishing of organic AM geometries. Find CNC machining providers on ForgedLink verified for tolerance capability, fixture design for AM parts, and integration with additive workflows.
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Why AM parts almost always need CNC finishing
As-built laser powder bed parts come off the printer with surface roughness of 6–12 µm Ra on vertical walls, 20+ µm Ra on down-facing surfaces, and dimensional tolerances of ±0.1 to ±0.2 mm — fine for many applications, but not for sealing surfaces, bearing fits, threaded holes, or features that mate to ISO H7 bores. The same is true for cast and forged parts, which is why investment-cast and forged parts have always been routed through finish-machining. AM does not change this: any part with mating surfaces, fasteners, or sealing geometry will pass through CNC machining as part of its production chain.
AM parts present some unusual machining challenges, though. The geometries are often organic, lightweighted, or topology-optimised — meaning standard fixturing strategies (vises, clamping flats) don't apply. Workpiece stiffness can be much lower than a billet equivalent, requiring lighter cuts and clever support fixturing. Print artefacts (residual stress, surface roughness, slight geometric distortion) demand probing-on-machine and adaptive toolpaths. Providers experienced specifically with AM-finishing typically run 3+2 or full 5-axis machining centres with on-machine probing (Renishaw OMP), printed sacrificial fixturing, and CAM workflows that adapt nominal toolpaths to the actual scanned-part position.
CNC operations commonly performed on AM parts
Bore and bearing-bore finishing
AM bores are usually printed undersized (typically 0.3–0.5 mm of stock) and finish-bored to ISO H7 / H6 fits. Reaming and honing are used for the tightest tolerance bores in hydraulic and bearing applications.
Sealing surfaces and O-ring grooves
O-ring grooves, gasket faces, and metal-to-metal sealing surfaces require Ra below 1.6 µm — much smoother than as-built. CNC turning or face milling brings these to drawing spec.
Threaded holes and fastener interfaces
Tapped or thread-milled holes for screws and bolts. As-built printed threads are not load-rated; production parts almost always have post-print machined threads or installed inserts (Heli-Coil, Ensat).
Mating flanges and assembly interfaces
Bolted flanges, alignment pins, and dowel-pin holes require flatness and positional tolerance that as-built AM cannot reliably hold. Standard practice is to print these features oversized and finish-machine after stress relief.
Turbine blade airfoils and aerodynamic surfaces
Five-axis finishing of LPBF / EBM turbine and impeller blades to bring as-built airfoils to aerodynamic profile tolerance and surface finish.
Build-plate witness removal and datum machining
After wire-EDM separation from the build plate, the part has a rough underside that often needs face-milling to establish a datum surface for subsequent operations.
Complex internal channel access ports
Drilling and tapping access ports into AM parts with internal cooling channels — heat exchangers, mould inserts, manifolds — for fluid connection or post-build powder removal.
Machining considerations by AM alloy
Ti-6Al-4V (LPBF / EBM)
Notoriously difficult to machine — low thermal conductivity, work-hardening, and chemical reactivity with cutting tools. AM Ti is slightly tougher than wrought due to the fine α′ structure; specify carbide tools, low surface speeds (40–60 m/min), and flood coolant.
Inconel 718 / 625
Among the toughest AM alloys to machine — high work-hardening rate and abrasion-resistant. Usually machined post-HIP and post-age, when properties are at peak hardness. Use high-pressure coolant, ceramic or coated carbide inserts, and rigid setups.
AlSi10Mg
Easy to machine compared to Ti and Ni — but the as-built fine cellular structure can produce unusually long swarf and built-up edge. Machine after T6 ageing for stable dimensions; AM AlSi10Mg responds slightly differently to milling than cast equivalents.
Stainless 316L / 17-4 PH
Standard machining alloys. 17-4 PH is best machined in the H1025 condition — H900 is harder and consumes more tool life. 316L works well in the as-built or solution-annealed state.
Maraging Steel (M300 / 1.2709)
Excellent machinability in the as-built state (~30 HRC) — providers typically rough-machine before age-hardening, then finish-machine after age (50–55 HRC) for tight features. Critical for conformal-cooled mould-insert workflows.
CoCrMo
Tough and abrasive. Machined post-HIP for orthopaedic implants. Specialised cutter geometries and rigid setups are essential — many medical-device CNC shops dedicate machines to CoCrMo work.
Polymer AM (SLS / MJF / SLA)
Polymer AM parts can be CNC-machined for tight tolerance features — typically with single-flute cutters at high speeds and air cooling. Used to add precise mating surfaces to functional polymer prototypes and end-use parts.
Designing AM parts for downstream CNC
Add machining stock at the design stage — typically 0.5–1.0 mm on faces that will be machined, 1.5–3 mm on surfaces that need significant material removal (e.g. flanges that need to be flat after stress-relief distortion). Print-stock allowance is the most common DfAM mistake — too little, and the as-built distortion eats the tolerance window.
Design printed datums into the part. A complex AM part with no obvious clamping surfaces is a fixturing nightmare. Adding sacrificial machining tabs, flat datum faces, or printed location features (cut off after machining) gives the CNC shop something to grip and reference from.
Sequence matters: print → stress relief → machine → HIP → age → finish-machine. Finishing critical features before HIP risks distortion during the high-temperature cycle. Finishing only after final age gives the most predictable result, at the cost of one extra setup.
Plan for on-machine probing. AM-experienced CNC shops probe each part on the machine to determine actual position vs nominal CAD, then adaptively offset the toolpath. This is essential because as-built parts do not sit on the fixture exactly where CAD says they will. Specify part-position verification as part of the routing.
Lead time and cost expectations for AM CNC finishing
Standard 3-axis or 3+2 finishing of typical AM parts (10–20 features) delivers in 1–2 weeks from receipt of the as-built part. Full 5-axis simultaneous machining of complex aerospace structurals can take 2–4 weeks due to fixturing development and CAM programming time. Repeat-order CNC work after first-article qualification runs faster, with most shops able to turn re-orders in 5–10 working days.
CNC pricing is typically charged at the shop's standard hourly rate (£60–£140 / €70–€165 per hour for 3-axis, £100–£220 / €120–€260 per hour for 5-axis), regardless of whether the workpiece arrived as-built from AM or as raw billet. Setup and fixturing development for first-article AM parts often adds 4–12 hours of NRE that amortises over the production run. For typical aerospace bracket-class AM parts, finish-machining adds 30–80% to the total part cost.
Related processes & materials
Frequently asked questions
Why can't AM produce final-tolerance features without CNC?
Because the physics of layer-by-layer melting impose hard limits on dimensional accuracy and surface finish. As-built LPBF tolerance is typically ±0.1 to ±0.2 mm with 6–12 µm Ra surface roughness — fine for many features, but well outside ISO H7 bore tolerance (±0.025 mm typical) and below the surface finish required for sliding seals or bearings. CNC finish-machining bridges the gap.
Should I use my regular CNC shop or a specialist AM-finishing shop?
For simple finishing operations (face milling, drilling, tapping), a competent general CNC shop is usually fine. For complex 5-axis finishing of organic AM geometries — turbine blades, lattice structures, topology-optimised brackets — an AM-experienced shop is worth the premium. They'll have probing-and-offset workflows, printed sacrificial fixtures, and CAM strategies tuned for low-stiffness AM workpieces.
How much machining stock should I add to AM features?
Rule of thumb: 0.5–1.0 mm per side on faces that will be machined to spec, 1.5–3 mm per side on surfaces that need post-stress-relief distortion correction, and 3–5 mm on surfaces that need substantial material removal (e.g. printed billet stock for downstream forging-replacement work). Always discuss with your AM and CNC providers — both will have view on what works for the specific geometry and alloy.
When should CNC finishing happen in the production chain?
Standard sequence is: print → stress relief → wire-EDM separate → rough machine (datums, gross stock removal) → HIP → solution + age → finish machine (critical features). Some providers compress this by combining stress relief with HIP-solution cycles. Finishing only after final age gives the most stable result, since dimensions don't shift further during downstream heat treatment.
Are 5-axis machining centres really needed for AM finishing?
Often — but not always. For organic, topology-optimised, or freeform AM geometries with features on multiple faces, 5-axis lets you machine all features in one setup, eliminating the cumulative position error of multiple 3-axis re-fixturings. For simpler AM parts with features on 1–2 faces, 3-axis or 3+2 setups are entirely adequate and cost less per hour.
Can polymer AM parts be CNC-machined?
Yes — SLS, MJF, SLA, and FDM parts can all be machined for tight-tolerance features (mating bores, flat reference surfaces, threaded inserts). Use single-flute cutters, high spindle speeds, and air cooling (no flood coolant — most polymers absorb moisture or react with cutting fluids). This is standard practice for end-use polymer parts that need a precision interface to a metal assembly.